In a largish pan, over a low to medium heat, add the treacle, honey and peanut butter. Stir continuously while the mixture comes to a slow boil.
Take the mixture off the heat, and add in around 10 marshmallows. Stir in well until they are fully melted in.
<span class="mceItemHidden" data-mce-bogus="1"><span></span>Mix the krispies and the cereal into the mixture well.
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Use the back of a spoon to press your mixture down into your tray. Mark the top of your tiffin into sections, pop a marshmallow on the top of each section, and leave to cool until serving.